As is well known in the art, electric vehicles operable by the power of a battery include a pure electric vehicle operable by the battery power only, and a hybrid electric vehicle that uses both a conventional internal combustion engine and a battery as power sources. A battery used for storing electric energy is provided for such an electric vehicle, and such a battery should be maintained in a proper state. For the purpose of the maintenance, detection of an output voltage of the battery should be enabled. A battery of an electric vehicle usually includes a plurality of modules (battery modules), and accordingly, a voltage of each of the battery modules should be observed in order to detect the output voltage of the battery. In order to properly maintain the battery, it is important to improve preciseness and accuracy in detection of the battery voltage.
One attempt at improving precision in detection of the battery voltage is described in a Laid Open Publication of Japanese Patent application No. 1996-292215, where the improvement of the precision is suggested to be achieved by enhancing resolution of an A/D converter that converts an analogue voltage signal to a digital signal. However, such a scheme does not help improve the accuracy in detection of the battery voltage, which is the motivation of the present invention. In order to measure the voltage of a battery module, a differential amplifier circuit having a differential amplifier is usually adopted. At least one resistor is provided in such a differential amplifier circuit, such that the differential amplifier circuit modifies a voltage difference inputted via input terminals on the basis of a gain according to resistivity of the resistor and outputs the modified voltage difference.
The word “amplify” or variations such as “amplifier” or “amplification”used in an expression such as “differential amplifier” should be understood to cover both the meanings of “increase” and “decrease” of a signal level. It is notable that an increase or decrease of a signal level may be obviously changed therebetween by changing the value of the gain, and the word “amplify” or its variation in the conventional expression “differential amplifier” is kept for better comprehension of the description and claims of the present invention.
A controller is connected to output terminals of the differential amplifier circuit, and the controller stores resistivity value of the resistor used in the differential amplifier circuit as a constant value.
Accordingly, when voltages of both terminals of a battery module are applied to the input terminals of the differential amplifier circuit, the differential amplifier circuit modifies the input voltage and outputs it to the controller. Then, the controller modifies the received voltage difference on the basis of the gain according to the stored constant resistivity value, and determines such recovered voltage difference value as an original voltage difference value received at the differential amplifier circuit, i.e., actual voltage of the battery module.
However, the resistivity value stored in the controller may have errors in comparison with actual resistivity of the resistor used in the differential amplifier circuit, thereby causing deterioration of accuracy in detection of the battery voltage. Such an error may be caused by tolerance of the resistor allowed in manufacturing of the resistor, or by temperature dependency of the resistivity of the resistor. That is, actual resistivity of a resistor may be different from the specification within an allowed tolerance. The actual resistivity may also be different from the specification because the actual resistivity depends on its operating temperature. Accordingly, voltage value of the battery obtained by the differential amplifier circuit may vary according to the operating temperature of the differential amplifier circuit.
The information disclosed in this Background of the Invention section is only for enhancement of understanding of the background of the invention and should not be taken as an acknowledgement or any form of suggestion that this information forms the prior art that is already known to a person skilled in the art.